Sintered rubbing contact material and method for producing same



' SINTERED RUBBING CONTACT MATERIAL AND METHOD FOR PRODUCING SAME FiledJan. 21, 1963 P 1965 R. J. M DONALD 3,205,565

F I G 2 INVENTOR.

ROBERT J. MocDONALD ATTORNEY nited tates Pate This invention relates toa sintered rubbing material and to the method for producing same and,more particularly, to an improvement of a novel rubbing materialdisclosed in copending application U.S. Serial No. 26,751, now US.Patent No. 3,081,196, Robert J. MacDonald, assigned to the same assigneeas this application. The material may be used for bearings, pistonrings, seals, clutch plates and the like.

The above noted application pertains to material formed by a matrix of ametal powder mixture consisting, generally, of a material whose freeenergy of oxide formation is more positive than lead oxide, e.g., copperor silver, and in which the matrix is infiltrated by particlespredominantly of lead oxide. It was noted in the said application that acomparatively large amount of predominantly lead oxide particles,combined with other oxides, are infiltrated into the matrix, to provideat the rubbing surfaces of the mating materials always a rathersubstantial amount of the predominantly lead oxide base particles. Byvirtue of the depth of the matrix material, the rubbing surface materialwithstands wear over a long period of time and still functions toprovide bearing material at the mating surfaces.

This invention avoids infiltration of the matrix by the lead oxide basematerials and thereby overcomes some of the problems inherentlyconnected with this approach. While an infiltrated matrix of a compositematerial here under consideration may be desirable, under certainconditions and for some applications, there are, nevertheless, others inwhich an infiltrated structure has distinct disadvantages.

Infiltration is a function of the viscosity of the mate rial, in thiscase molten oxide, a high viscosity melt, will not always suitablyinfiltrate the matrix material. More over, the binary lead oxide mixturehas a certain range of viscosity which depends upon the alloyingaddition. Thus, only certain mixtures readily infiltrate. Furthermore,infiltration is at times difiicult to control especially when the rateof infiltration is rather slow. Inasmuch as molten lead oxide readilydissolves copper, a prolonged surface contact can result in appreciablesurface erosion and structural weakening. An infiltrated structure mustby its very nature have inter-connecting areas of such lead oxideparticles which have a deleterious effect on the machinability of thecomposite material. This is to say, that large inter-connecting areas oflead oxide or alloys thereof are, under certain circumstances, readilypulled out by a machine tool, leaving voids of at least microscopicdimensions in the finished surface.

It has now been recognized that these conditions can be overcome bysolidly sintering a blended mixture of the matrix material and thepowder particles predominantly of lead oxide. In this improved articlethe powder particles of predominantly lead oxide form independent anddiscontinuous lakes which are embedded in the matrix material.

ice

It is therefore an object of this invention to provide an article of thetype here under consideration which has improved metallurgicalcharacteristics and establishes greater physical contact between theindividual metal powder particles.

It is another object of this invention to provide a unique method forproducing the improved articles.

It is another object of this invention to provide a material and amethod for producing such material in which the dispersion of the leadoxide or alloys thereof in the material is substantially improved so asto avoid pulling these particles out during the machining operation.

It is a still further object of this invention to provide a material anda method for producing same which is more compatible with the uniquerequirements of viscous oxide whereby a greater variety of lead oxidemixtures may be sintered with the matrix material than is normallypossible with the infiltration approach previously discussed.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

An aspect of the present invention resides in a method of forming asintered powder metal member which includes the steps of blending powderparticles selected from a group consisting of copper, silver and alloysthereof with powder particles consisting essentially of lead oxide, toestablish a uniformly distributed mixture. Thereafter the mixture ofpowder particles is compacted to green density and then the compactedmixture is sintered to metallurgically bond the individual powder particles together.

Another aspect of the present invention resides in the provision of amethod of providing a sintered powder metal lining upon a solidreinforcing metal back which comprises the steps of depositing anintermediate layer of bonding material upon a substantially flat surfaceof a continuous strip of non-porous metal. Then there are blended powderparticles of a material whose free energy of oxide formation is morepositive than lead oxide with powder particles predominantly of leadoxide and the resulting mixture is spread upon the fiat surface of thestrip facing the intermediate layer. Thereafter, the strip is passedthrough a sintering chamber which has conditions effective to sinter thepowder particles together for establishing merely a point contactbetween the particles and, moreover, to metallurgically bond theparticles to the fiat surface of the strip. The strip is then passedthrough a pair of pressure rolls to establish a predetermined density ofthe metal lining; the strip is then passed through a chamber which hasconditions therein effective to re-sinter the powder particles togetherat their contacting surfaces.

A further aspect of the invention resides in the provision of a sinteredpowder metal member formed from a metal powder mixture of a matrixmaterial whose free energy of oxide formation is more positive than leadoxide and powder particles predominantly of lead oxide, and in which thepowder particles of predominantly lead oxide form independent anddiscontinuous lakes in the matrix material.

A further aspect of this invention resides in the provision of acomposite metal member which comprises a steel backing member and anintermediate bonding layer coextensively bonded to the backing memberand wherein the bonding layer consists of a metal taken from the groupof metals whose free energy of oxide formation is more positive thanlead oxide and alloys thereof. The composite metal member also includesa solidly sintered bearing layer which is coextensively bonded to andoverlays the intermediate layer and is formed of a metal powder mixtureof a matrix material whose free energy of oxide formation is morepositive than lead oxide and in which the powder particles ofpredominantly lead oxide form independent and discontinuous lakes whichare embedded in the matrix material.

FIGURE 1 is a drawing made from a photo-micrograph, magnified 250 times,showing the metallurgical characteristics of the material of theabove-named copending application for comparative purposes only. Thereference numeral 10 designates the generally continuous lakes of thelead oxide base material. Where such continuity is not visible, thecontinuity exists in another plane. The light areas 12 in the FIGURE 1represents the matrix material and of course includes such voids.

FIGURE 2 is a drawing made from a photo-micrograph showing, magnified250 times, the metallurgical characteristics of the material of thepresent invention. There is shown in the drawing independent anddiscontinuous lakes of the lead oxide base material 14 which areembedded in the matrix 16.

Anessential quality for the matrix material is that the metal powdersforming such matrix have a free energy of oxide formation which is morepositive than that of lead oxide so that the matrix material does nottend to reduce the lead oxide to metallic lead when both of thematerials are co-sintered. Both copper and silver satisfy thisrequirement. Moreover, silver can also be used as an alloy ing elementto strengthen the copper without adverse effect to the lead oxide.Arsenic may be used as an alloying element, too, however it is notsatisfactory as a base metal.

In the following discussion, the terms copper and lead oxide areconsistently used for purposes of illustrating the invention. The extentto which other materials can be substituted for, or combined with, thesematerials has been delineated in other parts of this specification. Inno event, however, should the sole reference to copper and to lead oxidebe interpreted in a restrictive sense.

Almost any grade of copper powder is acceptable. However, one powderblend has been found to be particularly well suited because when mixedwith fine powder oxides it flows very readily. This is an importantproperty, especially for use with high speed compacting presses commonlyemployed in the powder metal industry. The nominal screen analysis ofthis preferred copper blend is as follows:

Nominal percent: Screen size 8 100+1s0 22 -150+2c0 9 20o+250 23 -250+3253s -325 Composites have been made with additions of lead oxide per se,as well as with lead oxide base mixtures. The strength of the compositematerial is primarily attributed to the copper matrix, whereas the oxideadditions provide friction wear qualities.

Following is Table I which gives a large number of lead oxide basemixtures and some pertinent information relating thereto which fallwithin the scope of the present invention. At the head of Table I isreference data for lead which are included for purposes of comparison.

4 Table 1 PbO-BASE MIXTURES Eutectic specific example, percent MeltingRange, percent Min. 5l Viscous.

Fluid.

Viscous.

Very Viscous.

Balance.

0. Very Viscous. Fluid.

Do. Very Viscous.

A proper balance must be obtained to produce a rubbing material havingthe best combination of rubbing qualities and mechanical strength. Witha copper matrix, it is believed that the addition of 5 weight percent ofa lead oxide base mixture, such as for instance PbO8SiO will provide asubstantial improvement in rubbing characteristics compared to unalloyedcopper. Above 20 weight percent of oxide the strength of the compositematerial and also the wear resistance during rubbing are reduced. Thebest oxide range for most applications appears to be in the area between10 and 15 weight percent.

The composite metal body is densified to a degree considerably short ofthe theoretical density so that the body may be impregnated with alubricant to provide a self-lubricating rubbing member if suchproperties are desirable. It has been found that a 10% porosity in acompacted matrix permits retention of some oil and allows some degree ofconformability within the material.

The process for fabricating the above-described composite material intoa compact and sintered member is described in two parts. The first partrelates to solid sintered composite structure per se, while the secondpart is concerned with a metal lining formed of this composite materialand bonded to a steel backing member. Fundamental to both methods is thesintering of the composite matrix material in the presence of lead oxideand/or alloys thereof. This factor is of paramount importance.

In the process for the composite structure per se, the matrix materialsare first thoroughly blended with a lubricant to coat the copperparticles to reduce the frictional forces which develop between theparticles and the die wall during the pressing operation. It is entirelypossible, however, to press these powders without the addition of alubricant. But, to prolong tool life and to permit a faster rate ofpressing, it is desirable that a lubricant be added to the blend. It hasbeen found best to blend the copper particles with stearic acid foronequarter hour before adding the oxide base mixture at which time theblending is continued for an additional three-quarters hour. By blendingfirst the copper particles and the lubricant, the lubricant has a betterchance tocoat the copper powder and the tendency for oxide lubricantballing is greatly reduced.

The blended mixture is then compacted by conventional automatic pressingdevices. The green density to which the materials are pressed is of someimportance. I have found a green density of 7.3 g./cc. to be the best.This corresponds roughly to about 83% of the theoretical density of thecomposite material. Densities in excess of 7.3 g./cc. have the tendencyto cause the oxide to bleed minutes are used at this temperature.

during the sintering process. This occurs because the oxides melt andexpand, and if this expansion cannot be taken up by internal porosity,the oxide will exude out of the compact. To achieve such a green densitya compacting pressure in the vicinity of 50,000 psi. is required.

The compacted mixture of the powder particles is then sintered in atwo-stage process.

The first step in the two-step sintering process constitutes apre-oxidation treatment to provide a film of cupric oxide around thecopper particles. Generally, this treatment is effected at a temperaturebetween 700 and 1000 F. A preferred temperature is 800 F. which appearsto provide a controlled oxidation Without prolonged temperatureexposure. Depending on the mass of the piece treated, times on the orderof 10 to 1 At higher temperatures, of course, it i possible to reducethe time element, however, temperatures substantially above 800 F. causecuprous oxide (Cu O) to be formed. This coating, however, is not astenacious as cupric oxide (CuO) and in some cases easily spalls from thecopper. This preoxidation treatment prior to sintering is desirable fortwo primary reasons: (1) it eliminates the free lead formed by thedecomposition of the powdered lubricant and (2) provides a surfacestructure having optimum rubbing characteristics.

With the addition of C11 0, the melting point of PbO is thereby loweredfrom 1630 F. to 1260 F. It appears that a similar relationship must alsoexist for the PbO-CuO system. When a film of CuO is present on eachcopper particle, sintering above 1260 F. results in a wetting of allcopper surfaces with PbO, or when previously alloyed, a PbO richmixture. Such a behavior allows the oxide to be at the immediatesurfaces of the part after sintering. Therefore, subsequent coining orsizing can make the part entirely usable without the need for machining.

The presence of a film of CuO also serves another function during thesintering process. Any metallic lead which forms due to the burning offof the powdered lubricant will react when molten with CuO and will causea Thermit reduction producing metallic Cu and PbQ. In this way, thepresence of Pb may be almost completely eliminated from the composite.

It is implicit in the above statement that the preoxidation treatment isnot a strict requirement for producing a composite of Cu and PhD.However, to permit fabrication by conventional powder metal means, andto produce an end material having optimum strength and rubbingproperties, this pre-treatment oifers considerable advantages.

The preoxidized compact is then sintered to strengthen the bondingbetween the copper particles and to obtain a bond between the PbO or PhDbase mixtures and the copper particles. While the temperature requiredfor sintering is not critical it has been found best to sinter above themelting point of the oxide addition. A temperature range of 1400 to 1700F. generally applies to the compositions of this invention. Thus, forPbO which melts at 1630 F., sintering may be accomplished between l650and 1750 F. On the other hand, for example, the PhD- 8Si0 binary alloy,which melts between l250 and 1350" F. sintering can be accomplished at1500 to 1550 F. The preferred holding time at the sintering temperatureto obtain reproducible strength property appears to be at around 15minutes, although this aspect is not too critical. Because PbO isreadily reduced to metallic Pb, sintering cannot be done using areducing atmosphere, such as hydrogen, endothermic, exothermic, ordissociated ammonia gases without post oxidation as noted below. Aninert gas such as argon or nitrogen is used. While normally thesintering of unalloyed copper in an inert atmosphere is not nearly aseffective as when a reducing gas is used, it should be noted that thisdifliculty is overcome by the addition of PhD and its alloys to Cupowder inasmuch as,

during sintering, some PbO will vaporize and actually combine with anyCuO or Cu O present, leaving a clean copper surface. Alternatively, thelead particles may be post oxidized to convert these particles to leadoxide by Thermit reaction with the copper oxide particles.

Upon sintering the material or post oxidation as aforesaid, it isdesirable to repress or coin the compact. This may be done using thesame tooling equipment used for compacting the original green compactsince the sintering process results in very little dimensional change.Normally, the same amount of pressure applied during the compacting stepmay also be used for coining the sintered compact. The specified densityof the final composite will depend upon the application to which thearticle will be employed. Where hydrodynamic lubrication conditionsexist and where a thin film of lubricant must be maintained, a materialnear full density is desired. A composite having about density withrespect to theoretical density (7.93 g./cc.) appears to be best for suchconditions. The 10% porosity remaining in this material allows theretention of some oil. The composite material, more particularly theporous portions thereof, are infiltrated with the lubricant by means ofconventional vacuum impregnation methods. It is obvious, where the finaldensity is decreased, the internal oil supply may be raised which makesit possible to provide a self-lubricating composite material.

The method of providing a sintered powder metal lining upon a solidreinforcing metal back, follows, generally, the basic concept of theinvention outlined above. To facilitate the bonding of the compositematerial to the steel backing member and to eliminate any undesirablereaction between PbO or PhD base mixtures and the steel backing member,the latter is plated with a bonding material that has the samecharacteristic with respect to the oxide formation as above described.Thus, generally, the steel member is plated with a copper or silveroverlay or an alloy thereof; a copper film within the range of .0005 to.002 inch is sufficient for most of the compositions above named.Greater thicknesses, however, may be necessary where materials of highermelting temperatures are utilized.

The powder blends of copper and oxide are prepared following theprocedure outlined above. No lubricant is added since the process stepsnecessitating such lubrication are absent. The blended powder mixture isthen spread upon the fiat surface of the strip facing the intermediatelayer. The strip is then passed through a sintering chamber which hasconditions therein to sinter the powder particles together to establisha point contact, i.e., limited cohesion, between the particles and tometallurgically bond the particles to the flat surface of the strip. Thetemperature for sintering the strip should normally be between 1400 and1700 F. for the various composi tions discussed herein. For example fora Cu-PbO-Si0 blend, a temperature of about 1500 F. appears to be bestsuited.

Ordinarily in the fabrication of steel backed bearings of, for instance,copper lead, babbitt, or aluminum material, final sizing is accomplishedby precision boring. Thus, the surface condition of the cast or sinteredstrip is not important. In some cases, however, it may be desirable toform a bearing from a strip stock and to prepare the final bore bysizing or roller burnishing techniques. In such an instance, it isdesirable to have a surface with optimum oxide distribution. For thisreason the free-sintered composite may be preoxidized, a treatmentcomparable to that employed in the aforedescribed method. it should benoted that this preoxidizing should, preferably, take place before therolling step.

The free sintered strip is now rolled to at least a 75% reduction todensity and to increase its bond strength.

The preferred range for densifying the sintered powder mixture isbetween 80% and of theoretical density. The strip is then re-sintered ata temperature between 1400 and 1700 F. to sinter and fuse the contactsof th particles together to strength and improve the bond. Uponre-sintering the strip the powder mixture may then be vacuum impregnatedwith a lubricant which fills the remaining pores. The finished strip isthen machined or finished by conventional methods to the desirablefinish and end structure.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is aimed,therefore, in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. The method of forming a sintered powder metal member for rubbingcontact, including the steps of: blending powder particles selected froma group of materials whose free energy of oxide formation is morepositive than lead oxide and consisting of copper, silver and alloysthereof with powder particles consisting essentially of lead oxide toestablish a uniformly distributed mixture; compacting said mixture ofpowder particles to green density; and sintering the compacted mixturein an inert atmosphere to metallurgically bond the individual powderparticles together.

2. The method of forming a sintered powder metal member for rubbingcontact, including the steps of: blending powder particles selected froma group of materials whose free energy of oxide formation is morepositive than lead oxide and consisting of copper, silver and alloysthereof with a lubricant for applying a protective coat onto saidparticles; blending the coated particles with powder particlesconsisting of a lead oxide mixture to establish a uniformly distributedmixture; compacting said mixture of powder particles to green density;and

sintering the compacted mixture in an inert atmosphere tometallurgically bond the individual powder particles together.

3. The method of forming a sintered powder metal member for rubbingcontact, including the steps of: blending powder particles selected froma group of materials whose free energy of oxide formation is morepositive than lead oxide and consisting of copper, silver and alloysthereof with powder particles consisting essentially of lead oxide toestablish a uniformly distributed mixture; compacting said mixture ofpowder particles to green density; preoxidizing the compacted mixture bysubjecting the mixture to a temperature effective to oxidize theparticles selected from the said group of copper, silver and alloysthereof; and then sintering the compacted mixture in an inert atmosphereto metallurgically bond the individual powder particles together.

4. The method of forming a sintered powder metal member for rubbingcontact, including the steps of: blending powder particles selected froma group of materials whose free energy of oxide formation is morepositive than lead oxide and consisting of copper, silver and alloysthereof with powder particles consisting essentially of lead oxide toestablish a uniformly distributed mixture; compacting said mixture ofpowder particles to green density; sintering the compacted mixture in aninert atmosphere to metallurgically bond the individual powder particlestogether; and thereafter oxidizing the sintered and compacted mixture byexposing the mixture to an oxidizing gas to re-oxidize by thermitreaction those lead oxide particles which have been reduced to lead.

5. The method of forming a sintered powder metal member for rubbingcontact, including the steps of: blending powder particles selected froma group of materials whose free energy of oxide formation is morepositive than lead oxide and consisting of copper, silver and alloysthereof with powder particles consisting essentially of lead oxide toestablish a uniformly distributed mixture;

compacting said mixture of powder particles to green density; sinteringthe compacted mixture in an inert atmosphere to metallurgically bond theindividual powder particles together; and then coining the sintered andcompacted mixture to final size, shape and density.

6. The method of forming a sintered powder metal member for rubbingcontact, including the steps of: blending copper particles, whose freeenergy of oxide formation is more positive than lead oxide, withparticles consisting essentially of lead oxide to establish a uniformlydistributed mixture; compacting said mixture of powder particles togreen density; pre-oxidizing the compacted mixture to provide a film ofcupric oxide around said copper particles by subjecting the mixture to atemperature of 700 to 1000 F.; and sintering the compacted mixture in aninert atmosphere to metallurgically bond the individual powder particlestogether.

7. The method of forming a sintered powder metal member for rubbingcontact including the steps of: blending powder particles selected froma group of materials whose free energy of oxide formation is morepositive than lead oxide and consisting of copper, silver and alloysthereof with powder particles of lead oxide and additives selected froma group consisting of: SiO A1 0 2 3 s, 2 5, 2 5 3 3, 2 3 2 3, TiO S P 0Fe O and Cu O; compacting said mixture of powder particles to greendensity; and sintering the compacted mixture in an inert atmosphere bysubjecting the mixture to a temperature above the melting point of thelead oxide when singularly present, and above the melting point of thelead oxide base mixture when said lead oxide is combined with one ormore of said additives, to metallurgically bond the powder particlestogether.

8. The method of forming a sintered powder metal member for rubbingcontact, including the steps of: blending powder particles selected froma group of materials whose free energy of oxide formation is morepositive than lead oxide and consisting of copper, silver and alloysthereof with a lubricant for applying a protective coat onto saidparticles; blending the coated particles with powder particlesconsisting essentially of lead oxide to establish a uniformlydistributed mixture; compacting said mixture of powder particles togreen density; pre-oxidizing the compacted mixture by subjecting themixture to a temperature effective to oxidize the particles of copper,silver and the said alloys thereof; and then sintering the compactedmixture in an inert atmosphere to metallurgically bond the individualpowder particles together.

9. The method of forming a sintered powder metal member for rubbingcontact, including the steps of: blending powder particles selected froma group of materials whose free energy of oxide formation is morepositive than lead oxide and consisting of copper, silver, and alloysthereof with a lubricant for applying a protective coat onto saidparticles; blending the coated particles with powder particles of leadoxide and additives selected from a group consisting of SiO A1 0 Bi OCrO V205, AS205, M003, W03, 3203, B203, Tioz, SnO P 0 Fe O and Cu O;compacting said mixture of powder particles to green density;pre-oxidizing the compacted mixture by subjecting the mixture to atemperature effective to oxidize the particles of copper, silver and thesaid alloys thereof; and thereafter sintering the compacted mixture inan inert atmosphere by subjecting the mixture to a temperature above themelting point of the lead oxide when singularly present, and above themelting point of the lead oxide base mixture when said lead oxide iscombined with one or more of the additives, to metallurgically bond theindividual powder particles together.

10. The method of providing a sintered powder metal lining for rubbingcontact upon a solid reinforcing metal back, including the steps of:depositing an intermediate layer of bonding material upon asubstantially flat surface of a continuous strip of nonporous metal;blending powder particles of a material whose free energy of oxideformation is more positive than lead oxide with powder particlespredominantly of lead oxide and spreading the resulting mixture upon theflat surface of said strip facing said intermediate layer; thensubsequently passing said strip through a sintering chamber havingconditions therein effective to sinter said powder particles togetherfor establishing merely a point contact between said particles and tometallurgical-1y bond said particles to said flat surface of said strip;thereafter passing said strip through a pair of pressure rolls toestablish a predetermined density of the metal lining; andthen-subsequently passing said strip through a chamber having conditionstherein efiective to re-sinter the powder particles together at theircontacting surfaces.

11. The method according to claim wherein the strip is passed throughsaid rolls to densify said compacted powder particle between 80 and 95percent of theoretical density.

12. The method of providing a sintered powder metal lining for rubbingcontact upon a solid reinforcing metal back, including the steps of:depositing an intermediate layer of bonding material upon asubstantially flat surface of a continuous strip of nonporous metal;spreading a loose layer of a copper powder base mixture composed ofmaterial whose free energy of oxide formation is more positive than leadoxide, including particles of lead oxide, upon the flat surface of saidstrip facing said intermediate layer; then subsequently passing saidstrip through a sintering chamber having an inert atmosphere and asintering temperature above the melting point of lead oxide to sintersaid powder particles together establishing a point contact between saidparticles and to metallurgically bond said particles to said fiatsurface of said strip; thereafter passing said strip through a pair ofpressure rolls to establish a predetermined density of the metal lining;and then subsequently passing said strip through a chamber havingconditions therein effective to re-sinter the powder particles togetherat their contacting surfaces.

13. The method of providing a sintered powder metal lining for rubbingcontact upon a solid reinforcing metal back, including the steps of:depositing an intermediate layer of bonding material upon asubstantially fiat surface of a continuous strip of nonporous metal;spreading a loose layer of a copper powder base mixture composed ofmaterial whose free energy of oxide formation is more positive than leadoxide, including in a minor amount powder particles of a lead oxide baseeutectic mixture, upon the flat surface of said strip facing saidintermediate layer; then subsequently passing said strip through asintering chamber having a sintering temperature above the melting pointof the lead oxide base eutectic mixture to sinter said powder particlestogether establishing a point contact between said particles and tometallurgically bond said particles to said flat surface of said strip;then subsequently passing said strip through a pair of pressure rolls toestablish a predetermined density of the metal lining; and thensubsequently passing said strip through a sintering chamber having asintering temperature above the melting point of the eutectic mixture tore-sinter the powder particles together at their contacting surfaces.

14. The method according to claim 13, in which the eutectic mixture iscomposed of P bO-SiO and the temperature applied for sintering saidmixture is between 1400 and 1700 F.

15. The method according to claim 13, in which the powder particles aresintered in a non-oxidizing atmosphere.

16. The method of providing a sintered powder metal lining for rubbingcontact upon a solid reinforcing metal back, including the steps of:depositing an intermediate layer of bonding material upon asubstantially fiat surface of a continuous strip of non-porous metal;blending a copper powder base mixture, whose free energy of oxideformation is more positive than lead oxide, with powder particlespredominantly of lead oxide and spreading the resulting mixture upon theflat surface of said strip facing said intermediate layer; thensubsequently passing said strip through a sintering chamber havingconditions therein to sinter said powder particles together establishinga point contact between said particles and to metallurgically bond saidparticles to said flat surface of said strip; pre-oxidizing the pointcontact sintered mixture by subjecting the mixture of particles to atemperature effective to oxidize the particles composed of said copperbase mixture to establish a film of cupric oxide around the particles;then subsequently passing said strip through a pair of pressure rolls toestablish a predetermined density of the metal lining; and thereafterpassing said strip through a chamber having an inert atmosphere andconditions therein to re-sinter the powder particles together at theircontacting surfaces.

17. The method according to claim 16, wherein said mixture ispre-oxidized at a temperature between 700 and 1000" F.

18. The method according to claim 13, wherein the metal lining of saidstrip while passing through said rolls is densified to a degree short oftheoretical density, and then, subsequent to the re-sintering stepfilling the remaining pores with a lubricant through vacuumimpregnation.

19. A sintered rubbing contact member formed from a metal powder mixtureof a matrix material whose free energy of oxide formation is morepositive than lead oxide and powder particles predominantly of leadoxide, said powder particles of predominantly lead oxide formingindependent and discontinuous lakes embedded in said matrix material.

20. A rubbing contact member substantially as in claim 19, furthercharacterized by said matrix material consisting essentially of copper.

21. A rubbing contact member substantially as in claim 19, furthercharacterized by said matrix material consisting essentially of silver.

22. A ru-b'bing contact member substantially as in claim 19, Eurthercharacterized in that said powder particles predominantly of lead oxideinclude at least one other oxide material.

23. A rubbing contact member substantially as in claim 22, furthercharacterized in that the last mentioned other oxide material isselected from a group consisting of Slog, A1203, Blgog, CIO3, V205,AS205, M003 W03, Sb2O B203, TiO S1102, P205, Fe O and C1120.

24. A sintered rubbing contact member formed from a metal powder mixtureof a cop'per powder matrix material and powder particles predominantlyof lead oxide; the said particles of lead oxide constituting at leastfive weight percent, but not substantially more than twenty weightpercent, of the total powder mixture, and said lead oxide particlesforming independent and discontinuous lakes embedded in said matrixmaterial.

25. A rubbing contact member substantially as in claim 24, furthercharacterized in that the composite mixture has a density of at least ofthe theoretical density providing a porous structure; and aninfiltrating lubricant within said pores.

26. A composite rubbing contact member comprising: a steel backingmember; an intermediate bonding layer coextensively bonded theretowherein said layer consists of a metal taken from the group of metalswhose free energy of oxide formation is more positive than lead oxideand alloys thereof; and a solidly sintered bearing layer coextensivelybonded to and overlaying said intermediate layer and formed of a metalpowder mixture of a matrix material whose free energy of oxide formationis more positive than lead oxide and powder particles predominantly oflead oxide, said powder particles of predominantly lead oxide formingindependent and discontinuous lakes embedded in said matrix material.

27. A composite rubbing contact member according to claim 26, furthercharacterized in that the intermediate bonding layer is composed of amaterial in which copper predominates on a per weight basis.

'28. A composite rubbing contact member according to claim 26, furthercharacterized in that the intermediate bonding layer is composed of amaterial in which silver predominates on a per weight basis.

29. A composite rubbing contact member according to claim 27, furthercharacterized in. that theintermediate layer has a thickness between.0005 and .002 inch.

30. A sintered rubbing contact member formed from a metal powder mixtureof a matrix material whose free energy of oxide formation is morepositive than lead oxide and powder particles of a lead oxide basemixture,

L2 said powder particles of said lead oxide base mixture formingindependent and discontinuous lakes embedded in saidlmatrix material.

References Cited by the Examiner UNITED STATES PATENTS 2,198,253 4/40Koehring 29-1825 2,200,855 5/40 Ruben 29182.5 2,372,202 3/45 Hensel29l82.5 2,831,243 4/58 Thomson 29-4825 3,019,514 '2/62 Bickelhaupt eta1. 29-182.5 3,026,200 3/62 Gregory 75224 CARL D. QUARFORTH, PrimaryExaminer. REUBEN EPSTEIIN, Examiner.

1. THE METHOD OF FORMING A SINTERED POWDER METAL MEMBER FOR RUBBINGCONTACT, INCLUDING THE STEPS OF: BLENDING POWDER PARTICLES SELECTED FROMA GROUP OF MATERIALS WHOSE FREE ENERGY OF OXIDE FORMATION IS MOREPOSITIVE THAN LEAD OXIDE AND CONSISTING OF COPPER, SILVER AND ALLOYSTHEREOF WITH POWDER PARTICLES CONSISTING ESSENTIALLY OF LEAD OXIDE TOESTABLISH A UNIFORMLY DISTRIBUTED MIXTURE; COMPACTING SAID MIXTURE OFPOWDER PARTICLES TO GREEN DENSITY; AND SINTERING THE COMPACTED MIXTUREIN AN INERT ATMOSPHERE TO METALLURGICALLY BOND THE INDIVIDUAL POWDERPARTICLES TOGETHER.
 30. A SINTERED RUBBING CONTACT FORMED FROM A METALPOWDER MIXTURE OF A MATRIX MATERIAL WHOSE FREE ENERGY OF OXIDE FORMATIONIS MORE POSITIVE THAN LEAD OXIDE AND POWDER PARTICLES OF A LEAD OXIDEBASE MIXTURE, SAID POWDER PARTICLES OF SAID LAD OXIDE BASE MIXTUREFORMING INDEPENDENT AND DISCONTINUOUS LAKES EMBEDDED IN SAID MITRIXMATERIAL.